(1) Field of the Invention
The present invention relates to screw elements for multi-screw extruders with pairs of co-rotating and fully wiping screws, to the use of these screw elements in multi-screw extruders and to a method of generating these screw elements.
(2) Description of Related Art
Co-rotating twin- or multi-screw extruders whose rotors fully wipe each other have been known for a long time (see, for example, German Patent No. 862,668). Screw extruders based on the principle of fully wiping profiles are used for many diverse applications in the field of polymer production and processing. This is mainly due to the fact that polymer melts adhere to surfaces and are degraded over time at the processing temperatures commonly employed. This is prevented by the self-cleaning effect of fully wiping screws. Rules for generating fully wiping screw profiles are described for example in Klemens Kohlgrüber: Der gleichläufige Doppelschneckenextruder (“The co-rotating twin-screw extruder”), Publishers: Hanser Verlag, Munich, 2007, pp. 96 et seq.), in which it is also stated that a predefined screw profile on the 1st shaft of a twin-screw extruder determines the screw profile on the 2nd shaft of a twin-screw extruder. The screw profile on the first shaft of the twin-screw extruder is therefore referred to as the generating screw profile. The screw profile on the 2nd shaft of the twin-screw extruder is based on the screw profile on the 1st shaft of the twin-screw extruder and is therefore referred to as the generated screw profile. In multi-screw extruders the generating screw profile and the generated screw profile are always arranged alternately.
Modern twin-screw extruders consist of a modular system in which various screw elements can be mounted onto a central shaft. This allows those skilled in the art to adjust twin-screw extruders to suit the processing task concerned.
The screw elements known according to the prior art are—with the exception of eccentrically arranged circular discs—characterized in that the cross-sectional profile contour has at least one bend (see, for example, FIG. 1) occurring at the transition between the screw tip and the flight flanks. The tip consists of an arc with a radius equal to the outer diameter of the profile and a centre point at the point of rotation of the profile. The bend at the transition to the flank of the profile forms a crest on the screw element.
One of the main processes performed in multi-screw extruders is the dispersion of liquid phases or melts which cannot be homogeneously mixed with each other or the dispersion of solids in polymer melts. It is known from the technical literature (see, for example, Chang Dae Han: Multiphase Flow in Polymer Processing, Academic Press, New York 1981), that a combination of shear and elongational flow is the best method of solving difficult dispersion problems.
Such a type of flow prevails in a screw channel in which the material is not only sheared by the rotation of the screws but is also simultaneously elongated by the convergence of the screw channel towards the tip. In the region of the screw tip only shear flow does however prevail, which is hardly conducive to solving difficult dispersion problems. On the other hand, most of the energy introduced is dissipated in the gap between the screw tip and the barrel and between the screw tip and the adjacent screw, which is why this region plays a major role in the heating of and thus in potentially thermally damaging the polymer composition, without making any contribution to the processing task of dispersion.
Eccentrically arranged circular discs, which are known to be capable of being arranged in a fully wiping fashion, are an exception. They do not have a tip region exclusively producing shear flow. They are well-known for their excellent dispersing action, although they also create a high input of energy since they form a very narrow gap over a large circumferential region. They are also restricted to a flight number Z of 1.
Based on the prior art, the problem therefore arose of providing screw elements for multi-screw extruder machines which have improved dispersing action compared to the prior art and require only a low input of energy.
Surprisingly it has been found that this problem is solved by screw elements whose profile contours are continuously differentiable over their entire cross-sections.